1. Field of the Invention
The present invention relates to a method of taking an action for positively inducing arc generation if an arc is not generated between a wire head and a workpiece to be welded where an arc welding torch is mounted on an industrial robot to attempt to perform arc welding for the workpiece to be welded.
2. Description of the Related Art
In executing an arc welding for a subject of welding (hereinafter referred to as a workpiece) using an industrial robot (hereinafter simply referred to as a robot) on which an arc welding torch is mounted, an arc discharge sometimes fails to be generated even when a welding torch and a workpiece are brought close to each other while a welding voltage is supplied.
If such a situation occurs, it is necessary to immediately take an action for inducing an arc generation, and then the robot is allowed to move along a welding line. Such an action taken for inducing an arc generation, hereinafter referred to as a retrial motion, will be explained in the following. As the retrial motion, the following two methods have been known:
(1) A method, in which the surface of the workpiece is scratched by the head of a welding wire if no arc is generated at a point when welding is started.
In the retrial motion of this method, as shown in FIG. 1, a head 4 of a wire 3 is moved from a welding start point P1 to a point P'1 on a midway of welding route ranging from the welding start point P1 to the welding end point P2, while being pressed against the top surface of a workpiece W. The movement of the wire head 4 is performed by a robot (of which only an arm head 1 is shown in FIG. 1) supporting a welding torch 2. During the movement of the head 4 of the wire 3 from the point P1 to the point P'1, the slug and the like causing the failure of arc generation are removed. The head 4 of the wire 3 when reached the point P'1 leaves the workpiece W and is returned to the welding start point P1. If an arc still fails to generate even after one retrial motion, the same motion will be repeated until an arc is generated. Then, when an arc is generated at last, the head 4 of the wire 3 is returned once to the welding start point P1 to start arc welding.
(2) A method, in which the wire 3 is repeatedly moved back and forth with respect to the workpiece W by sending out the welding wire 3 from the welding torch 2 and withdrawing it thereinto using a wire feeder.
In the retrial motion by this method, as shown in FIG. 2, the robot supporting the welding torch is at rest, so that the welding wire 3 alone is sent out from the welding torch 2 to cause its head 4 to approach or contact the welding start point P1, or withdrawn into in the welding torch 2 to cause its head 4 to leave the welding start point P1.
When an arc is generated between the wire head 4 and the workpiece W by the trial motion in the above-mentioned method (1) or (2), the length of welding wire 3 to be sent out of the welding torch 2 is adjusted, aria the robot is allowed to move from the welding start point P1 to the welding end point P2 (hereinafter, the movement of the robot means the movement of a tool center point corresponding to the position of the head 4 of the welding wire 3 which has been sent out by a predetermined amount from the welding torch 2 supported on the arm head 1 of the robot), thereby performing welding in the span from the point P1 to the point P2 on the workpiece.
However, in the above-mentioned method (1), the wire 3 is continuously fed even while the robot is moving, so that a situation is apt to be brought about in which the wire head 4 is hardly moved despite the fact that the robot is moving. If such a phenomenon occurs in which the movement of the wire head 4 is retarded, the motion of the wire head for scratching the surface of workpiece is not performed effectively, so that the effect of the action for inducing arc generation becomes weak as a result.
Also, in the above-mentioned method (2), a sagging of the wire 3 is apt to occur when the welding wire is sent out from or withdrawn into the wire feeder 6, and so there is a problem that it is difficult to control the wire feeder 5 so that the wire head 4 is made to optimally approach and leave the surface of the workpiece. That is, this method has a disadvantage in that, in this method, the performance of the retrial function depends on the performance of an external apparatus, so that the inherent ability of the welding system is unable to be fully utilized.